The long term goal of the proposed research in this application is to determine the selective basis of host cell response (sensitivity or resistance) to antiviral dideoxynucleosides (ddN). Prolonged therapy of AIDS patients with anti-HIV ddN, in addition to selecting drug resistant HIV-1 variants, may induce cellular resistance because of their dependence on the host cells' activating enzymes. The research effort in this area remains extremely limited. The Principal Investigator observed that chronic exposure to 0.5 ZM dideoxycytidine (ddC) in vitro induced drug resistance H9 lymphocytic cells (H9-ddC cells). These cells compensated for impaired mitochondrial functions and had lower deoxycytidine kinase (dC kinase) activity and ddCTP levels. The aim of this proposal is to study biochemical basis of the ddC resistance and whether such a mechanism operates in the patients undergoing ddC therapy. Therefore, the following specific aims will be investigated (i) mechanisms of the reduced dC kinase activity and ddCTP levels; (ii) compensatory mechanisms for impaired mitochondrial functions in the resistant cells; (iii); differences in ddC resistance from that of its analog arabinosyl-cytosine (ara-C); and (iv) development of cellular resistance in the patients treated with ddC. These goals will be achieved by studying kinetics and genetic expression of dC kinase gene, cellular nucleotide pool, cross resistance to other nucleoside analogs, and mitochondrial DNA content structure and number per cell H9, H9-ddC, CEM, CEM-araC cells and the lymphocytes and autopsy specimens from the patients who had been treated with ddC. It is expected that these studies will enhance understanding of the host cell determinants responsible for ddC antiviral activity and may lead to strategies for improved chemotherapy.